Polysaccharides and carbohydrate-containing components of various hemoflagellates of the parasitic family Trypanosomatidae have been studied. Polysaccharides isolated from a number of species of the genus Crithidia have been separated and characterized as previously reported for Crithidia fasciculata. Relationships between the organisms within this genus have been established by chemical analysis of these constituents. A circulating antigen from mice infected with Trypanosoma cruzi, previously identified with a carbohydrate containing antigen extracted from culture forms (epimastigotes) of Trypanosoma cruzi, has been quantitated and shown to be proportional to the parasitemia in the mouse. Evidence for the loss or shedding of this antigen from trypanosomes in vitro has so far been negative. Hexose and glycerol uptake by Leishmania tropica have been investigated. Sugar and glycerol phosphorylating activities were characterized with respect to several kinetic parameters, substrate specificity, cofactor requirements and effects of inhibitors. The results indicate a very close relationship between transport and the phosphorylation step. The uptake and utilization of amino acids and other metabolites by Entamoeba histolytica is being studied. The intracellular fate of these substrates is being determined.